The invention relates to a distance meter, in particular to a system and a method for signal acquisition in a distance meter.
Distance meters of this generic type are well known in the prior art. They have a distance measuring range of several tens of meters and are often embodied as a handheld units. They are used primarily in construction surveying or in indoor construction, for instance for measuring rooms three-dimensionally. Other areas of application for distance meters are geodetic and industrial surveying. The fundamental principle of distance measurement by the known instruments is based on evaluating a change over time of a parameter of the electromagnetic beam emitted by the instrument and remitted by an object aimed at. To that end, the distance meter is equipped with an emitter for emitting an intensity-modulated beam. In handheld instruments, this primarily involves an optical beam in the visible wavelength spectrum, to make it easier to aim at the measurement points. The optical beam is remitted or scattered by the measurement object aimed at and is recorded by a receiver built into the instrument. From the time lag of the received modulated beam compared to the beam emitted by the emitter, the distance from the measurement object is found.
As detectors, PIN photodiodes or avalanche photodiodes are typically used in the known distance meters for converting the beam remitted or scattered by the measurement object into electrical signals. Distance meters whose distance determination is based on the measurement principle of phase measurement are very widely used. In such instruments, the electrical reception signal has a control frequency superimposed on it, directly at the avalanche photodiode or after a preamplifier, to make a low-frequency measuring signal. The phase of this low-frequency signal is determined and compared with the phase of a reference signal. The difference between the measured phase of the low-frequency measuring signal and the phase of the reference signal is a measure of the distance of the measurement object.
From German Patent Disclosure DE-A 196 43 287, a method and a system for calibrating distance meters are known. In particular, this reference describes how, when an avalanche photodiode is used, a stable reference phase can be generated to enable guaranteeing the measurement accuracy even in the presence of various environmental factors and equipment-dictated factors. Compared to other known photodiodes, such as PIN photodiodes, avalanche photodiodes have approximately 100 times the gain and thus have a correspondingly greater sensitivity. To achieve this high gain, they require markedly higher operating voltages in operation, and this is highly dependent on the working temperature of the avalanche photodiode. The operating voltage is applied as a bias voltage to the avalanche photodiode and is moreover individually different from one photodiode to another. Avalanche photodiodes are produced in a multi-stage, highly specialized semiconductor process. Additionally integrating circuit components by which the requisite bias voltage is stabilized and reduced to an expedient level would make the production process even more complicated and would increase the already high costs for avalanche photodiodes still further. The requisite high bias voltage and the increased power consumption prove especially disadvantageous for portable handheld instruments, which are operated with batteries or accumulators. The greater number of conventional batteries requires larger housings that are not as handy. Moreover, the readiness for use of these battery-operated instruments is relatively brief. Using special batteries or accumulators also affects the size and handiness of the handheld instruments and moreover increases the price.
An object of the invention is to create a distance meter which can have lower power consumption. The housing of the instrument can be kept small, so that particularly in handheld instruments, handiness is assured. The costs for producing the instrument can also be kept low.
An exemplary system for signal acquisition in a distance meter includes at least one photoelectric receiver, which detects an electromagnetic beam modulated at high frequency via a modulation frequency, and converts it into high-frequency electrical signals, and a device for transforming the high-frequency electrical signals, furnished by the photoelectric receiver, into low-frequency measuring signals, which can be carried onward for evaluation to a signal-processing unit mounted downstream. The device for transforming the high-frequency electrical signals, furnished by the photoelectric receiver, into low-frequency measuring signals includes, according to an exemplary embodiment, at least one switch, whose switching frequency is controlled by a control frequency whose frequency is greater or less, by the frequency of the low-frequency measuring signal, than the modulation frequency. The switch, which can be actuated at high frequency, is connected to a downstream capacitor, which is adjoined by a transimpedance amplifier at whose output, in operation, the low-frequency measuring signal is present.
In an exemplary circuit arrangement, the output of the photoelectric receiver is applied directly to a switch. The high-frequency electrical signal is carried to a capacitor by the switch that is also switched at high frequency. The switching frequency of the switch is slightly greater or less than the modulation frequency of the emitted electromagnetic beam. A transimpedance amplifier is coupled to the capacitor, and the charge collected at the capacitor is carried away to it. In this way, the voltage at the capacitor remains practically nearly constant. A low-frequency measuring signal is applied to the output of the transimpedance amplifier and is evaluated in the usual way in the downstream signal-processing unit. The switch that can be operated at high frequency replaces the relatively complex mixer, known from the instruments in the prior art, in which the high-frequency electrical signals furnished by the photoelectric receiver have a control frequency superimposed on them in order to generate low-frequency measuring signals from the high-frequency signals. The switch is operated by way of the applied control frequency in such a way that the same half-wave of the high-frequency electrical signal is always switched through to the capacitor. From the capacitor, the charge switched through is carried onward, already at low frequency, to the transimpedance amplifier. In contrast to the known embodiments, a high-frequency electrical signal therefore need not be (pre-)amplified. This has the great advantage that, in contrast to the known versions, the transimpedance amplifier does not have to have a bandwidth in the range of several hundred megahertz, with a resultant transimpedance of only a few kiloohms. On the contrary, in an exemplary embodiment, transimpedance amplifiers for low-frequency signals with a bandwidth of only a few kilohertz suffice. With them, transimpedance that are greater by a factor of up to 103 can be achieved. Given the fact that for medium ambient brightness, it is precisely the preamplifier of the measuring signal that is dominant in terms of noise in distance meters, the attainable improvement in the signal-to-noise ratio can be immediately appreciated. In the transimpedance amplifiers used, the thermal noise of the feedback resistance is definitive. The signal level at the output of the transimpedance amplifier rises linearly with the feedback resistance; the noise at the output, however, increases only in proportion to the square root of the feedback resistance. The transimpedance amplifiers with a relatively narrow bandwidth that can be used by circuit arrangements of the invention have precisely especially high feedback resistances and accordingly lead to a marked improvement in the signal-to-noise ratio. In this way, even with simple PIN photodiodes, signal-to-noise ratios can be attained that are equivalent to those when avalanche photodiodes are used. On the other hand, the known disadvantages of avalanche photodiodes are avoided.
To improve the signal levels, it proves advantageous to provide the sequential arrangement, of the switch controlled via the high-frequency control frequency, the downstream capacitor, and the transimpedance amplifier, in dual form. The two switches are switchable in alternation with the control frequency. As a result, both half-waves of the high-frequency electrical signal are added together at the output of the photoelectric receiver and are each delivered to one transimpedance amplifier. The outputs of the two transimpedance amplifiers are connected to the inputs of a differential amplifier that unites the two inverse, low-frequency measuring signals. With this provision, the signal-to-noise ratio can be improved still further.
An exemplary circuit arrangement furnishes the preconditions for integrating the signal acquisition system in a semiconductor component. Different scales of integration are possible. In each case, it is expedient to embody the high-impedance switches as field effect transistors. An integrated semiconductor component made with CMOS technology can be used. The transfer gates that can be achieved with this technology have the requisite high switching speeds and are distinguished by low power consumption and low supply voltages. In an advanced stage of integration, the capacitors and the downstream transimpedance amplifiers are disposed on the integrated semiconductor component. With the resultant shortening of the signal paths, particularly of the high-frequency signals, the broadcasting is also reduced. It is even possible for the circuit arrangement for generating the control frequency to be integrated with the semiconductor component, in order to avoid stray factors by means of the separate course of high-frequency signal paths in the instrument. The low power consumption of the integrated circuit arrangement proves advantageous for battery or accumulator operation of the distance meters. Moreover, there is less waste heat in operation, which allows a higher packing density of the components and has a favorable effect on the size of the handheld instruments.
The distance meters of this generic type are based on the principle of phase measurement. It proves advantageous if the receiver circuit required for a reference signal path has a structure analogous to the measuring signal path and is likewise disposed on the integrated semiconductor component. As a result of the integrated arrangement on the same semiconductor component, the calibration problems are elegantly solved. The receiver circuits for the measuring signals and for the reference signals are optimally coupled thermally and have a symmetry that with a conventional design can hardly be achieved.
In a further stage of integration, the photoelectric receiver for the measuring beam and optionally a separate photoelectric receiver for a reference beam can also be integrated on the semiconductor component. An exemplary circuit arrangement of the invention and the use of CMOS technology make it possible to perform this integration step as well, without major difficulties. Because of the further integration of the receiver arrangement, an even greater packing density is attainable, and the size of the handheld instrument can be reduced.
The integrated embodiment of the photoelectric receiver offers the possibility in a very simple way of embodying its active face in a segmented way. For instance, the photoelectric receiver can be equipped with at least two, and preferably three or more, receiver segments that are activatable independently of one another. This structural provision takes appropriate account of the fact that the measuring light spot that strikes the active face of the photoelectric receiver increases in size at a transition from long distances to shorter distances and also shifts in its position. This is a consequence of the projection properties of the input optical element of the distance meter. By the segmentation of the active face of the photoelectric receiver, this effect can be compensated for, and complicated additional optical elements that are vulnerable to flaws can be dispensed with.
In known distance meters, effort and expense is undergone so that the optical components and the photoelectric receiver are adjusted exactly. An integrated embodiment of the photoelectric receiver offers the possibility of providing adjustment aids.
To that end, at least one auxiliary photodiode embodied in integrated form is disposed in the vicinity of two edges, extending perpendicular to one another and adjacent one another, of the active face of the photoelectric receiver for the measuring beam. The auxiliary photodiodes, equipped with a simple downstream electronic detector, offer a simple capability of adjustment.
An integrated embodiment of a photoelectric receiver for the measuring beam and optionally a further photoelectric receiver for the reference beam on the semiconductor component also can offer a simple capability of providing optical filters in an integrated design. The integrated design in turn can have the great advantage that the optical filters for the photoelectric receivers for the measuring beam and for the reference beam have identical properties, because of the homogeneity of the production method.
The modulation frequency of the electromagnetic beam emitted by the distance meter is also determining for the control frequency with which the switch is operated. For the sake of attaining an especially good signal-to-noise ratio, control frequencies greater than 100 MHz prove advantageous.
A distance meter on the principle of phase measurement that is equipped with the system for signal acquisition of the invention can be distinguished by a simplified layout. It can have lower power consumption and can be operated with a lower supply voltage. This is advantageous especially for handheld devices, which because of the design according to the invention can also have a smaller housing.